Communication systems are increasingly constructed to permit the utilization of digital communication techniques by which to communicate information between a sending station and a receiving station. In a radio communication system, the communication channel is formed of a portion of the electromagnetic spectrum, i.e., the "bandwidth" allocated to the communication system.
A cellular communication system is a type of radio communication system and is exemplary of a type of communication system which is increasingly constructed to utilize digital communication techniques.
By utilizing a digital communication technique, the information of a communication signal can typically be more efficiently communicated between the sending station and the receiving station. In a radio communication system, the bandwidth allocated to the radio communication system is typically limited. The improved efficiency provided by the utilization of a digital communication technique permits the allocated bandwidth to be utilized more efficiently. By utilizing a digital communication technique, the communication capacity of such a radio communication system can sometimes be increased. In radio communication systems, the communication capacity of the system is limited by the allocated bandwidth. In a multi-user, radio communication system, for instance, an increase in the communication capacity permits additional users to communicate by way of the communication system.
A radio frequency link forming a communication channel between a sending station and a receiving station of a radio communication system is typically not an ideal, loss-free communication channel. A communication signal might be susceptible to degradation caused by multi-path fading. If significant, such fading might prevent the accurate recovery at the receiving station of the informational content of at least the portions of the communication signal subjected to such fading.
To increase the probability that the informational content of a digital communication signal transmitted by the sending station can be recovered once received at the receiving station, the data bits which are modulated to form the communication signal are sometimes encoded according to an encoding technique. Coding of the signal increases the redundancy of the signal. Even if portions of the communication signal are so distorted as to prevent some of the data bits modulated thereon to be recovered, the increased redundancy introduced by encoding the data bits increases the probability that the informational content of the signal can be recreated at the receiving station.
Various block and convolutional coding techniques have been developed to increase the redundancy of the signal at a sending station. Corresponding block and convolutional decoding techniques have similarly been developed to decode the communication signal, once received at the receiving station.
In at least one type of block coding technique, check bits are concatenated to blocks of data bits of which the communication signal is to be formed. The check bits are of values dependent upon the values of the data bits of such blocks of data.
In at least one type of convolutional coding technique, a coded sequence is formed of the data bits. The values of the bits of the coded sequence are dependent upon not only the bit values of the data bits which are to be encoded but also upon bit values of preceding bit sequences of data bits previously encoded.
Encoding of the data bits which are modulated to form a communication signal advantageously facilitates the recreation of the informational content of the signal when the interference introduced upon the signal is of short duration. If, however, the interference introduced upon the communication signal is of a lengthier duration, e.g., greater than several bits, encoding of the data bits does not ensure that the informational content of the signal shall be able to be accurately recreated.
Various interleaving techniques have been developed to reduce the possibility that interference introduced upon a communication signal during its transmission upon a communication channel shall prevent the recovery of the informational content thereof.
When the data bits are interleaved, consecutive data bits of the communication signal are "spread-out" so as not to be transmitted consecutively. Once the communication signal is received at the receiving station, the data bits are recombined. Because the data bits are spread-out over time, distortion is less likely to distort the consecutive bits in a manner to prevent the recreation of their informational content, once received at the receiving station.
Digital communication techniques are utilized in various cellular communication systems. For instance, a cellular communication system constructed pursuant to the standard specification of the Global System for Mobile communications (GSM) utilizes a digital communication technique. And, a cellular communication system constructed according to the EIA/TIA IS-95 specification, a CDMA (Code Division Multiple Access) system similarly utilizes digital communication technique. Prior to transmission of communication signals generated during operation of such systems, the data bits, of which the communication signals are formed, are encoded and interleaved. In a CDMA-based system, modulation is typically preceded by spreading of the coded and interleaved bits by a code sequence. Corresponding despreading is performed at a receiver. Operational protocols for the encoding and interleaving of data bits are also set forth in the respective standard specifications. Corresponding decoding and deinterleaving protocols are also set forth.
Although encoding and interleaving of the data bits of a communication signal increase the possibility that the informational content of the communication signal, subjected to interference during its transmission upon the communication channel, can be recreated, such encoding and interleaving, causes signal transmission delay. Interference may be caused, e.g., by distortion due to noise and both adjacent- and co-channel interference. In a CDMA-based system, interference can be caused from other users. The corresponding decoding and deinterleaving, causes additional signal transmission delay. If extensive, the transmission delay can also interfere with the quality of communications between a sending station and a receiving station.
When the radio communication system is utilized to transmit data rather than speech information, radio link performance is of increased significance. For instance, a bit error rate of 10.sup.31 3 is normally acceptable when the communication signal is formed of speech information. However, when data forms the informational content of the communication signal, a bit error rate performance of better than 10.sup.-6 is instead sometimes required.
Such a level of radio link performance requires additional encoding and interleaving of the data bits of a communication signal to be transmitted. However, if there is a correspondent increase in the signal transmission delay caused as a result of the additional encoding and interleaving, the resultant signal delay might be unacceptably large.
Utilization of a multi-stage encoding and interleaving technique permits the radio link performance to be improved. However, conventional multi-stage encoding and interleaving techniques typically introduce unacceptably large signal transmission delay.
A manner by which the radio link performance can be improved without causing a corresponding increase in the transmission delay would be advantageous.
It is in light of this background information relating to digital communication techniques that the significant improvements of the present invention have evolved.